X-ray protective circuits



May 14, 1957 K. A. KIESEL 2,792,503

X-RAY PROTECTIVE cIRcuI'rs Filed Sept. 4. 1953 Timer $1 me L ag I AVA AIAVA s7 68 48, CF; "E

0 R7 46 \49 WITNESSES: i INVENTOR Kenneth A.Kiesel.

X-RAY PRGTECTEVE CERCUITS Kenneth A. Kiesel, Baltimore, Md, assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of lenusyivania Application September 4, 1953, Serial No. 373,443

3 Claims. (Cl. 250-103) My invention relates to X-ray apparatus, and more particularly to X-ray protective apparatus.

It is usual practice in the X-ray art to utilize a milliampere stabilizer circuit to maintain X-ray tube current constant. Such stabilizers ordinarily vary the X-ray tube filament voltage to compensate for changes in X-ray tube current due to various factors. It is also usual practice to provide some types of X-ray tube overload protection, as for example, means to prevent the completion of an exposure when the combination of milliamperage, kilovoltage, and time set would exceed the X-ray tube rating. I have found, however, that there is yet another condition, for which no adequate protection has been thus far provided. During the operation of an X-ray tube, should the milliamperage decrease, the voltage between the cathode and anode of the X-ray tube will increase. If the X-ray tube is being operated at or near its maximum kilovoltage rating, a small decrease in milliamperage may cause the kilovoltage to exceed the tube rating, and a large decrease in milliamperage can cause the kilovoltage to increase suihciently to damage the tube. Normally, the milliamperage stabilizer will compensate for changes in X-ray tube current, thus maintaining the kilovoltage within safe limits. However, should the milliampere stabilizer itself fail, or should the drop in milliamperage be so rapid that the stabilizer cannot react fast enough to compensate, or should the X-ray tube filament circuit become defective, then the X-ray tube kilovoltage could well increase sufficiently to damage the tube. Further, during therapy treatment, the above conditions would result in incorrect radiation dosage, even if the tube were not damaged.

It is accordingly an object of my invention to provide for protection against the deleterious effects of sudden decreases in X-ray tube current.

It is another object of my invention to provide means for protecting an X-ray tube against damage which could result from a sudden decrease in X-ray tube current when the X-ray tube is being operated at or near its maximum kilovoltage rating.

It is another object of my invention to provide protection for an X-ray tube against decreases in X-ray tube current which for any reason are not compensated by an X-ray tube milliampere stabilizer.

It is another object of my invention to provide protection against the delivery of incorrect radiation dosage due to sudden decreases in X-ray tube current which are for any reason not compensated by a milliampere stabilizer.

These and other objects are effected by my invention as will be apparent from the following description taken in accordance with the accompanying drawing, forming a part of this application, in which the single figure is a schematic circuit diagram showing a preferred embodiment of my invention.

My invention may be advantageously utilized in conjunction with a circuit, such as that shown and described in U. S. Patent 2,512,193, issued to C, T. Zavales, and

2,792,503 Patented May 14, 1957 such a circuit will be used as a basis for the description of a preferred embodiment of my invention.

in the drawing there is shown an X-ray-tube 5 having its anode 6 connected through a rectifying valve 7 to one end of the split secondary winding 8 of a high tension transformer 9, with the remaining end of this secondary Winding 8 grounded at 10. In a similar manner the filamentary cathode 12 of the X-ray tube 5 is con nected through a rectifying valve 13 to one end of the remaining portion of a secondary winding 14 of the high tension transformer 9. The other end of this secondary winding connects through various instrumentalities constituting the stabilizer as will be hereinafter explained.

The filamentary cathode 12 of the X-ray tube 5 receives heating current from the secondary winding 15 of a low tension transformer 16. An auto-transformer 17 is provided with its primary being adjustably connected to a source of supply of the customary commercial potential of 200 to 250 volts so that once adjusted for the available voltage, upon closure of a magnetically operated switch 18, this voltage will be supplied to the autotransformer 17. It will be noted that the auto-transformer is provided with large steps throughout most of its length comprising several turns of its winding between taps, for the purpose of giving a voltage differential therebetween of ten volts, while the remainder of its winding is tapped at substantially each individual coil, for giving a voltage variation between taps in units. An additional winding 19 is carried by the core of the autotransformer 17 which is also tapped to give a voltage variation in units therebetween.

An adjustable arm 20 is operable to engage the various taps of the unit section of the auto-transformer for selecting the desired units of kilovoltage for the X-ray tube 5 which arm is connected to one end of the additional Winding 19. One end of the primary winding 22 of the high tension transformer 9 is adjustably connected to the additional winding 19, so that the adjustment may be altered to compensate for voltage drops in the interconnecting cable, drops due to leakage reactance, and auto-transformer drop at each value of millamperage and change in line voltage conditions. The other end of this primary winding 22 is connected upon closure of a magnetically operated switch 23, to an adjustable arm 24 engageable with the large ten volt taps of the autotransformer 17. The arms 29 and 24, are used to preselect the desired kilovoltage of the X-ray tube accurately in units and tens or 20 kv. and 2 kv. steps. The magnetic winding of switch 23 receives electrical energy from taps of the auto-transformer 17 upon closure of the pair of contacts 25, the pair of contacts 197, and the contacts of the X-ray On switch, with the circuit to winding of switch 23 remaining closed for a period of time determined by a timing device 26. Contacts 25 are closed by a relay 27, as hereinafter described.

The primary winding 23 of low tension heating transformer 16 receives energy of the proper voltage from autotransformer 17 by having one end of its winding connected directly to a tap of the auto-transformer winding, while the remaining end of primary winding 28 is adjustably connected by an arm 29 to a resistance 30 bridged across several windings of the auto-transformer 17, with a resistance 31 and the A. C. windings 33 and 34 of a saturable reactor 35 being in series with the primary winding 28 of filament heating transformer 16.

The saturable reactor 35 is provided in order to vary the filament heat of the cathode 12 to compensate for any inverse variations thereof due to decrease in line voltage, change in kilovoltage or change of resistance in the filament circuit by heating of its components. In addition to its A. C. windings 33 and 34 disposed about the outermost legs of the core, such reactor is provided with a D. C. winding 36 wound about its center leg. As is well known in the art, a change in magnitude of the current flowing in the central D. C. winding 36 will vary the impedance of the two A. C. windings 33 and 34, i. e., an increase in current through the D. C. winding will decrease the effective impedance of the two A. C. windings 33 and 34 allowing more current to flow to the primary of transformer 16 with an attendant increase in filament heat and a decrease in current through the D. C. winding 36 will increase the impedance of the two A. C. windings with a decrease in filament heat of cathode 16.

If, therefore, the magnitude of the current flowing through the D. C. winding 36 is suitably controlled, the filament heat of the filamentary cathode 12 isconveniently varied to maintain substantially uniform heat. Accordingly, an electronic control tube 37 is provided which has a relatively high value of transconductance rather than a high voltage gain. By high value of transconductance is meant a tube, such as identified in the trade as 6AG7, which will vary the eticctive plate current in the greatest possible manner with the least possible change in grid voltage, since this tube type has the characteristics desired in a control circuit which is dependent on current rather than voltage as a means of operation. Accordingly, the plate of tube 37 is connected to one end of D. C. winding 36 while its indirectly-heated cathode is connected to one side MA-l of the input circuit of the stabilizer and to the negative side of a D. C. source of supply of about 350 volts, as hereinafter described, and the opposite end of the D. C. winding 36 is connected to the positive side of this D. C. source of supply. The supply of a D. C. current at a voltage of about 356 volts to the D. C. winding 36 of the saturable reactor 35 is thus under the control of tube 37. A capacitor 4-1 is shunted across the D. C. winding 35 to reduce the effects of self-induction which occurs due to rapid changes in plate current through the tube 37.

Referring now to high tension secondary winding 14 of transformer 9, it will be noted that an end thereof is connected, through a milliammeter 33 and shunted by a capacitor 39, to terminal MA-l of a milliamperage preselector shown generally within the dot-dash frame 4%). The latter constitutes a resistance type of control which increases or decreases the X-ray tube filament current, thus raising or lowering the milliamperage flowing through the high voltage circuit of the X-ray tube. At the same time this milliamperage preselector adjusts the current at which the stabilizer operates.

This milliarnperage preselector comprises resistors R1, R2, R3, R4, together with previously mentioned resistance 39 connected across the taps of auto-transformer 17. Resistors R1 to R4 select the magnitude of the voltage which is applied to the milliamperage stabilizer across the terminals G1 and MA-l. Resistance 39 controls the filament heat of the X-ray tube in such manner as to ,increase the available filament voltage as the milliamperage is raised, with its adjustable arm 29 being operable in tandem with the adjustable arm 42 of resistor R1.

These two adjustable arms 29 and 42 are operable from the front of the control panel and are employed in conjunction with a precalibrated milliarnperage scale. For the sake of accuracy the high and low ends of the milliamperage selector scale is made to track with the selector pointer, which is assured by the resistors R2, R3 and R4. Adjustable resistor R2 is provided to set the upper or high milliamperage to which the selector is adjusted while adjustable resistor R3 and fixed resistor R4 adjust the lower milliampere value on the precalibrated scale. Once these are set for a given installation, they need rarely be further adjusted with the simultaneously operable resistor R1 and the variable resistance 3ilpreselecting the milliamperage desired. a

. This milliamperage preselector' 40 operates by being supphedwrth current from the milliampere circuit of the high tension transformer winding 14 through milliammeter 38 and resistors R2 and R1 to ground through ad-' iustable arm 42, and since the upper end of adjustable resistor R1 is connected to stabilizer terminal G1 and the lower end of resistor R2 is connected to stabilizer terminal MA1, a definite control voltage is impressed across the milliampere stabilizer. A capacitor C1 is shunted across the resistors R1 and R2 to insure proper tracking of the calibrated milliamperage scale due to wave-form changes which occur in the milliampere circuit at higher values of X-ray tube current. As previously mentioned, resistors R3 and R4 are primarily employed to trim the low milliampere scale of the precalibrated selector, but they also play a second role. Should resistor R2 become open-circuited, due to any particular surge, resistors R3 and R4 insure the application of the control voltage to the G1 and MA-l terminals of the stabilizer.

Also in order to protect capacitor C7. from deleterious surges or open-circuit of resistors R1 or R2, a safety circuit is included in the milliamperage preselector. This safety circuit includes two "oltage regulator tubes 43 and 44 with said tubes being connected in series with each other across the stabilizer terminal MA-1 and ground. Resistors R6 and R7 shunt the respective tubes 43 and to equally divide the voltage thereacross. These tubes accordingly are so arranged as to break down should the voltage across capacitor C1 exceed about 300 volts. These voltage regulator tubes 43 and 44- are connected or interlocked into the X-ray On push-button circuit, as indicated by the legend, by virtue of the safety contacts contained within the tube base. In other words, two pins in the voltage regulator tube base are connected together and the two tubes connected in series with each other and connected to the X-ray On circuit at terminals 45 and 46 extending from the tubes 43 and 44. In addition to protecting capacitor C1 from overvoltage, the tubes 43 and 44 protect the entire equipment in the sense that they must be inserted in their respective sockets to render the control system operative.

In order to remove the alternating current components of the tube milliamperage due to the superimposing of alternating current on the D. C. rnilliamperage of the X-ray tube, a filter circuit is provided in the stabilizer. Such filtering is accomplished by the provision of two series connected inductances 47 and 48 included in series to terminal G1. These inductances are of rather high value of approximately 700 henries each to enable the removal of as much A. C. ripple as possible without causing a great time constant circuit which would result in instability. In addition a tuned filter in the form of a capacitor 49 is shunted across the inductance 48 to remove any possibility of the 60 cycle component getting through inductance 47 due to any distributed capacitance existing in this inductance 47. An additional capacitor 59 is connected in series with inductances 48, 47 between the terminals MA1 and G1 to further filter the circuit and across which the output voltage of the filter circuit appears. This filter circuit thus removes the current component due to cable capacitance of the conductors and current due to the distributed capacitance in the high tension windings and accordingly applies to the stabilizer a voltage which is only dependent on the actual tube current.

The stabilizer is also provided with what is termed a neon discriminator which comprises a neon tube 52 having one electrode connected in series with a fixed resistance 53 and variable resistance 54 to the terminal MA-1 and the other electrode. connected in series with inductances 48, 47 to the terminal G1. The purpose of this neon discriminator isto provide further ampli flcation for proper control of the stabilizer system. Since the neon tube 52 is a cold cathode tube and so constructed that the voltage drop across the tube is constant, regardless of current passing through the tube, the addition of resistance 53 and variable resistance 54 will regulate the division of voltage.

This may be better appreciated by assuming certain values of voltage such, for example, as 100 volts being impressed across the neon lamp and the end of variable resistance 54. Due to the constant voltage drop characteristic of the neon lamp 52, the voltage drop thereacross will approximate 70 volts, leaving a voltage drop of approximately volts across the resistances 53 and 54. If now the voltage across the terminals is increased from 100 volts to 110 volts, the voltage drop across the neon lamp still remains at 70 volts but increasing the voltage drop across the resistances 53 and 54 to volts. Thus, only a ten percent increase in input voltage has occurred, but the voltage drop across resistances 53 and 54 has increased thirty-three percent, thereby giving an approximate gain in the amplification factor of three which facilitates the need for voltage gain in the X-ray tube circuit. Tue magnitude of the current flowing in the neon discriminator circuit is controlled by the resistances 53 and 54 with the adjustability of the latter enabling closer tuning of the filter circuit consisting of the inductances 47 and 48 together with the capacitors 49 and 59 by effectively clipping the peak of the voltage wave present at higher kilovoltage while maintaining the milliamperage constant for all practical purposes at all kilovoltage settings.

The output of the neon discriminator circuit connects to the control grid of electronic control tube 37 through a battery and a resistance element 56. This battery 55 is provided to select the operating point with regard to the characteristic curves of the tube 37, and is not subject to normal deterioration eifects since no current is taken from the battery 55 itself during operation, with the battery merely adding to the output voltage of the neon discriminator circuit, thereby varying the plate current in the control tube 37.

In order to supply the 350 volts D. C. to the D. C. winding 36 of the saturable reactor 35, a power transformer 57 is provided which has its primary Winding 58 connected to appropriate voltage taps of auto-transformer 17. The high voltage secondary winding59 delivering approximately 350 volts has its center tap connected to the cathode of the control tube 37 (and thus to one side MA-l of the stabilizer input potential) while the ends of this secondary winding 59 connect to the respective anodes of a full wave rectifier tube 60.

The thermionic cathode of rectifier 60 connects to one side of the D. C. Winding 36 of the saturable reactor 35. This D. C. supply for reactor winding 36 is provided with a choke input filter comprising an inductance and capacitor 66. Accordingly, the capacitor 66 is charged each half wave of the alternating current cycle in well known manner since by virtue of the center tapped secondary winding59, its respective halves are 180 out of phase, and the resulting D. C. output voltage appears across one end of the D. C. winding 36 and the cathode of control tube 37 as above noted. The rectifier tube 69 being of the thermionic cathode type requires a nor mal delay period of approximately 20 seconds before the tube reaches its normal operating temperature. Advantage is taken of this delay characteristic, as hereinafter described, in order to provide a desired time delay prior to operation of the control by giving the tube 60 an opportunity to arrive at its proper operating temperature.

For the purpose of presetting the filament heat in the X-ray tube circuit, to approximately the normal operating range as will be required for the particular milliamperage selected and to also make the milliamperage stabilizer independent of line voltage variations, a preset circuit including a voltage regulator tube 67 is provided for controlling the voltage supplied to the screen grid of control tube 37. This preset circuit includes the voltage regulator tube 67 having its cathode connected to one side of capacitor 66 and thus to the negative side of the DC. output of the previously described filter cir-' cuit which includes the capacitor 66. The anode of voltage regulator tube 67 is connected through a resistance 68 to the opposite or positive side of the choke filter circuit and hence to one side of the D. C. winding 36, and such anode is also connected through an adjustable resistance 69 to the screen grid of control tube 37.

As hereinbefore mentioned, the control tube 37 has such characteristics that if the screen-grid voltage is held constant the plate current through the control tube 37 is relatively independent of line voltage and hence variations in the latter have substantially no efiect on the plate current. However, for the purpose of controlling the screen grid voltage of control tube 37 to enable the filament of the X-ray tube 5 to be properly preset at the termination of a given exposure, a relay 70 is provided. The winding of this relay is controlled by the X-ray On control circuit. This relay 7% is provided With a pair of contacts 72 which upon closure short-circuit the adjustable resistance 69 and also provided with a set of contacts 73 connected to the motor of timer 26 so that the latter operates as soon as the X-ray tube is energized.

In the absence of provision to the contrary, operation of the milliamperage stabilizer is dependent on the magnitude of the voltage applied to the control grid of the control tube 37. Accordingly, when the voltage is removed from the control grid of tube 37, the grid bias will then drop to the value of the bias battery 55. This will increase the plate current in tube 37 and decreases the effective impedance of the saturable reactor 35 and the filament heat of the X-ray tube will be increased, which produces an undesirable condition in that current through the X-ray tube reaches relatively high values upon resumption of another exposure. It is to prevent such undesirable result that provisions are made to control the voltage on the screen grid of control tube 37.

Prior to initiation of an X-ray exposure, the relay 70 is inoperative and the screen grid is supplied with a voltage through resistances 68 and 69. Since the voltage drop across these resistances are additive, a voltage somewhat lower than that required for normal operation is supplied to the screen grid of control tube 37, and with this reducedscreen-grid voltage the plate current in the control tube is reduced. Under conditions of reduced plate current, the impedance of the saturable reactor 35 will be raised thereby lowering the filament heat of the X-ray tube cathode as long as X-rays are not being produced.

When the X-ray On push button is operated, relay 70 is energized and closed its contacts 72, which thus short-circuits the resistance 69, as previously mentioned, and raises the voltage supplied to the screen grid of control tube 37 to its normal operating voltage of about volts. This immediately raises the plate current of control tube 37 and reduces the reactance of the saturable reactor 35, which in turn increases the filament heat of the X-ray tube cathode during the exposure and until the latter is terminated by the timing device 26 or by depression of the X-ray OE push button. Thus the voltage regulator tube, together with its circuit as above described, performs the desired function of maintaining relatively stable operation of the milliamperage stabilizer regardless of line voltage fluctuations and provides a desirable preset characteristic for the X-ray filament circuit by automatically reducing the filament heat of the X-ray tube cathode at the termination of an X-ray exposure and raising the filament heat upon initiation of a given exposure.

As hereinbefore mentioned, the control tube 37 is preferably one having a high value of transconductance so as to give optimum performance in a milliamperage type stabilizer. Such high value of transconductance enables the milliamperage stabilizer in general to have sufiicient gain to overcome efiects of increased filament voltage,

increased temperature in the filament circuit which affects filament resistance and increased kilovoltage. However,

because ofthe fact that a high gain is obtained, a feed-' back condition arises between the milliamperage stabilizer and the X-ray tube. This condition causes a rapid oscil lation in the milliammeter circuit and undesirable characteristics are obtained. In using a control tubehaving a high value of transconductance to obtain its high voltage again, as above noted, it would be impossible to successfully compensate for the effects of increased filmment voltage, increased temperature in the filament circuit which afiects filament resistance and increased kilovoltage without overcoming this feedback condition.

To accomplish the latter, a feedback circuit is provided comprising a feedback transformer 74-, the primary winding 75 of which is connected across the resistance 1 included in series with the primary winding 28 of the heating transformer 16 for the cathode 12 of X-ray tube and the A. C. windings 33 and 34 of reactor 35. The primary winding 75 of the feedback transformer is thus subjected to the voltage drop across the resistance 31 which is thus proportional to the current flowing in the primary winding of the low tension cathode heating transformer 16. This voltage across primary winding 75 is boosted by a center tapped step-up secondary winding 76 which is connected to a fullwave rectifier circuit comprising a rectifier 77, capacitor 78 and resistor 79. The output from the step-up winding 76 is accordingly impressed across resistor 79 with the filter capacitor 78 shunted across the resistor 79 to add sufiicient time duration to the voltage pulses appearing across the resistor 79 which averages out over a half cycle since the constants or" this capacitor 78 and resistor 79 are so chosen that the voltage duration is of a second.

The voltage which appears across resistor 79 is in turn capacitatively coupled to the previously mentioned resistance element 56 through a capacitor 80. Under normal conditions, that is a condition in which oscillations do not occur in the X-ray tube milliamperage and hence in the filament heating circuit for the X-ray tube cathode, the voltage drop across resistor 79 remains constant and is blocked by capacitor 80 from in any way affecting the control grid of control tube 37. I On the other hand, should oscillations occur in the filament circuit and in turn the milliamperage of the X-ray tube, a pulsating voltage appears across the resistor 79. When this occurs, this rapidly changing voltage is transmitted by capacitor 30 to the control grid of tube 37, thereby reducing the magnitude of the oscillations. This is accomplished by selecting the polarity of the voltage across the capacitor 8"!) so that a negative pulse is supplied to the grid of he control tube 37 Whenever oscillations develop in the filament circuit, since the application of such negative pulse to the grid immediately result in dampening of the oscillations in the filament circuit.

As previously referred to, advantage is taken of the time delay characteristic of the cathode of rectifier 69 in reaching an electron emitting temperature to provide an additional safety feature which enables the entire system to be properly conditioned before an attempt is made to operate the apparatus to produce X-rays. To this end, the previously mentioned relay 27 has its winding connected across the capacitor 56 and is thus subjected to the voltageacross the latter.

Accordingly, since the voltage across capacitor 66 does not appear until the expiration of the time'delay required by the cathode of rectifier 643 to reach an electron emitting temperature with attendant charging of the capacitor 66, the winding of relay 27 is not energized until the voltage across the capacitor 66 reaches substantially full value. 7

When, however, this voltage rises to sufficient magnirude after the time delay,-the winding of relay 27 becomes energized and closes its previously mentioned contacts 25." Closure of these latter. contacts 25 -ac-,

cordingly conditions the X-ray 011" control circuit for operation upon depression of the X-ray On push button, since the control circuit is then completed except at the pointof closure by the latter. Upon depression of the X-ray On" push button, the winding of relay 7%.

becomes energized by completion of the X-ray On control circuit and closes its pair of contacts 72 and 73. When contacts 72 are thus closed, the resistance 69 is short-circuited to raise the voltage supply to the screen grid of control tube 37, as previously described, and simultaneously therewith closure of contacts 73 completes a circuit to the timer 26 starting the latter in operation.

At the same time depression of the X-ray On push button completes a circuit to the winding of magnetically operated switch 23 energizing the latter with attendant closure of its contacts 32 and 83. Contacts 32 accordingly cause energization of the primary winding 22 of high tension transformer 9 and the generation of the X-rays for the period of time for which the timer 26 has been preset, or until the X-ray Off push button is depressed, while closure or" contacts 83 completes a holding circuit for the winding of magnetically operated switch 23, thus enabling the X-ray On push button to be immediately released following initial depression thereof. Also contacts 33 complete a holding circuit for the winding of relay 70 so that the contacts 72 and 73 of the latter maintain the short-circuit condition of resistance 69 and the timer 2-6 in operation, until the exposure is completed by opening of the normally closed contacts of the timer 26, or opening of. the control circuit by depression of the X-ray Olf push button. It can thus be seen that the X-ray On push button is there fore inoperative to complete the X-ray On control circuit to energize relays 23 and 7 which thus precludes the generation of X-rays, until relay 27 is energized after expiration of the above noted time-delay and closure of its contacts 25.

By reference to the drawing, it will be noted that the X-ray On control circuit receives its energy from taps on the auto-transformer 17. For example, a conductor 84 extends from a tap of auto-transformer 17 to one side of winding of magnetically operated switch 23 and this same conductor also extends to one side of the winding of relay 70, as well as to one contact thereof. The other side of this control circuit extends from another tap of the auto-transformer 17 and by a conductor 85 through the pins of the voltage regulator tubes 43 and 44 in series and through the contacts 25 of the relay 27 and then througlrnormally closed X-ray Oil push button, the normally closed contacts of the timer 26, and to one side of the X-ray On push buttons as well as one of the contacts 83 of magnetically operated switch 23.

This control circuit is thus initially open at the contacts 25 and the normally open X-ray On push button. Upon initial energization of the auto-transformer 17 the primary winding 58 of power transformer 57 is energized With power from this source being supplied to condenser 66 with attendant energization of the winding of relay 27 following the above noted time-delay period. This accordingly closes the above traced X-ray On control circuit at the contacts 25 of relay 27, thus leaving the control circuit interrupted only at the point of contact of the X-ray On push button so that upon momentary depression of the latter, the X-ray On control circuit is completed in the manner above described.

It will also be noted that the magnetically operated switch 18, which may be considered as the main line switch, is energized by depression of push button 36,

contacts are closed, two of such sets completing the commercial source of supply to the auto-transformer 17, while the other set of contacts completes a holding circuit to the winding of magnetically operated switch 18. When this main line switch is closed, the thermionic cathodes of the various tubes are heated, including the thermionic cathode of rectifying valves 7 and 13, although the spec iic heating source therefore is not shown for simplicity. Once the main line switch 18 has been closed, the entire apparatus is then in condition for operation in the manner above described and after completion of an exposure, the main line switch is deenergized when desired by depression of a normally closed push button 87.

The circuit as thus far described, is substantially the same as the one shown and described in the aforementioned patent. Referring again to the drawing, the undercurrent protective circuit comprises a multi-electrode relay tube 103 having its anode 104 connected in series with the coil 105 of a relay 1%, which may be termed the undercurrent protective relay, the safety pins 101 of regulator tube 67, and choke coil 65, to the positive output terminal of the rectifier 60. The undercurrent protective relay 1436 has a pair of normally closed contacts 107 connected in series with the X-ray On control circuit. The cathode 1&8 of the relay tube 103 is connected in series with a variable resistor 109 to the negative output terminal of the rectifier 60. The relay tube 103 has its control electrode 110 connected to the negative output side of the milliampere stabilizer at one terminal of the resistor 53. The screen grid 111 of the relay tube 103 is connected in series with two voltage dropping resistors 112, 68, and the choke coil 65 to the positive output terminal of the rectifier 60. The first dropping resistor 112 is shunted by normally open contacts 114 of a delay relay 115, which has its actuating coil 116 connected in parallel with the coil of relay 70. A constant voltage appears across the output terminals of the milliampere stabilizer as long as the correct milliamperage is flowing through the X-ray tube. This voltage will remain constant for any milliampcrage setting made by the milliamperage selector, since as the milliamperage is increased the resistance of the selector control is decreased. The control grid 110 of the relay tube 193 derives a bias voltage from the milliampere stabilizer output, and this bias voltage will remain constant so long as the milliamperage is correct. However, after the selector control is set, a decrease in m-illiamperage will cause a decrease in voltage at the output terminals of the stabilizer, since the resistance of the selector control remains fixed. Small changes in milliamperage are immediately taken care of by the stabilizer, and the undercurrent protective relay 106 will not be actuated, but when the decrease in milliamperage is so great and/ or so rapid that the stabilizer cannot compensate for it, then the undercurrent protective circuit functions. Under normal conditions, the relay tube 103 is conducting slightly, but when the milliamperage decreases, causing a decrease in the voltage at the stabilizer output terminals, the relay tube control grid 110 becomes less negative with respect to its cathode 108 and the tube becomes more conductive. The relay tube current will then energize the coil 105 of the undercurrent protective relay 106 to open the normally closed contacts 107 in the X-rayOn circuit, thus opening the X-ray tube voltage supply circuit. The variable resistor 109 in the cathode circuit of the relay tube is used to adjust its bias, thus providing a means for causing the protective circuit to operate upon any desired predetermined decrease of milliampere stabilizer signal voltage. The variable resistor 109 will always be adjusted so that any milliamperage decrease that cannot be compensated for by the stabilizer will immediately actuate the undercurrent protective circuit. The voltage dropping resistors 112, 68 in the screen grid circuit of the relay tube function to prevent operation of the undercurrent protective circuit during the period when the X-ray apparatus is being warmed up and the desired techniques are being set. Upon termination of this period, the first dropping resistor 112 is shunted by actuation of the delay. relay 115. With the first dropping resistor shunted, the voltage on the relay tube screen grid 111 is such as to allow operation of the undercurrent protective circuit. The coil 116 of the delay relay 115 is energized by the closing of contacts 25 of the relay 27. The coil of relay 27 is energized after a certain delay period as has been hereinbefore described.

While I have shown my invention in only one form, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various changes and modifications without departing from the spirit thereof.

I claim as my invention:

1. The combination of an X-ray tube having an anode and a thermionic cathode, a source of heating current for said cathode, an excitation circuit connected to supply anode-cathode operating potential to said X-ray tube, including an alternating current voltage source and a high tension transformer having its primary winding connected to said source and its secondary circuit connected to supply voltage to said X-ray tube, a first circuit means connected to said secondary circuit and including an impedance member subject to variations in potential thereacross in response to variations in X-ray tube current, stabilizing means responsive to said variations in potential and associated with said source of cathode heating current to cause cathode heater current to vary inversely with changes in X-ray tube current over a predetermined range of magnitudes, switch means connected to control energization of said primary winding and a second circuit means connected to said first circuit means and responsive to said variations in potential and operable upon a decrease in X-ray tube current to cause actuation of said switch means to deenergize said excitation circuit.

2. The combination of an X-ray tube having an anode and a cathode, an excitation circuit for said tube including a high tension transformer having a primary winding and having a secondary winding connected to be traversed by the discharge current through said X-ray tube, a heating circuit for said cathode, a variable impedance device in said heating circuit to control the amount of heating energy supplied to said cathode, first circuit means connected to said secondary winding for deriving an electrical quantity proportional to variations in discharge current through said tube, second circuit means responsive to said quantity for controlling said variable impedance device to cause the cathode heating energy to vary inversely with variations in said discharge current over a predetermined range of discharge current variations thereby tending to stabilize said discharge current against variations, switch means connected to control energization of said primary winding, and third circuit means connected to said first circuit means and responsive to said quantity and operable upon a decrease in said discharge current beyond a predetermined magnitude to cause actuation of said switch means to deenergize said excitation circuit.

3. The combination of an X-ray tube having an anode and a cathode to be heated, a source of alternating current energy for heating said cathode, a high tension transformer having its primary winding connected to a voltage source and having its secondary winding connected to supply anode-cathode operating potential to said X-ray tube, first circuit means connected to said secondary winding so as to be traversed by the discharge current through said X-ray tube and including an impedance member subject to variations in potential thereacross in response to the magnitude of X-ray tube current, stabilizing means responsive to said variations in potential and associated with said source of heating current to cause said heating to cause actuation of said switch means to deenergize' said excitation circuit upon a predetermined decrease in X-ray tube current.

References. Cited in the file of this patent UNITED STATES PATENTS SiIbermann Sept. 24, 1940 Gates Feb. 8, 1949 Zavales June 20, 1950 Beach et a1 Jan. 2, 1951 

